1. Field of the Invention
This invention relates to a modified polyurethane foam and a post-treatment process for producing the same.
2. Description of the Prior Art
It has long been known that it is not possible to generate an explosion of any fuel/air mixture in the presence of reticulated polyurethane foam structures. Because of this characteristic of reticulated polyurethane foam and the additional benefits of light weight, minimal reduction in a fuel tank's usable space and attenuation of violent surging of fuel, reticulated polyurethane foam structures have found extensive use in military aircraft and land vehicles. These properties reduce the vulnerability of these vehicles by providing protection against explosive ballistic hits, explosions following fuel tank rupture, explosive static discharge and explosions causes by lightning.
Unfortunately, the use of foam can give rise to a different danger. Since the foam has a low electrical conductivity, (or as as more conveniently measured, a high electrical resistivity) then there can be a buildup of electrostatic charges and the electrostatic charges can result in sparking, leading to a fire or explosion within the non-foam containing areas of the fuel tank. The build up of electrostatic charges can also adversely effect sensitive aircraft instrumentation.
Consequently, it is an object of this invention to provide a foam for fuel tanks which minimizes the possibility of fuel explosion from gunfire, electric ignition and lightning. It is a further object of this invention to provide a foam which has a higher electrical conductivity needed to minimize the danger of sparking caused by the buildup of electrostatic charges. It is a further object of this invention to provide fuel tanks which minimize the release of fuel in the event of rupture and which resist sparking due to the buildup of electrostatic charge.
It is also an object of this invention to provide a process for the modification of the properties of a polyurethane foam.
One process for the modification of polyurethane foam is treating the foam with a chemical additive so that the chemical additive coats the surface of a foam. Such processes typically involve spraying, padding or melting material onto polyurethane foam. The disadvantages of adapting such a process to the formation of foam for use in fuel tanks include the tendency of the chemical additive to either flake off the foam into the fuel, thus providing particles which can lead to fuel line blockage or an increased chance of a fire or to leach out of the foam and become dissolved in the fuel.
A second process for the modification of polyurethane foam is incorporating a chemical additive into the polymer itself by the so-called in situ technique. Incorporation can be achieved "in situ" by mixing a chemical additive with polyurethane foam-forming reactants. The chemical additive is mixed with the foam-forming reactants during the reaction, and consequently the chemical additive becomes intimately mixed with the subsequently formed polymer.
The disadvantages of in situ incorporation include the possibility that the desired chemical additive, or a carrier for the chemical additive, will react with the polyurethane foam-forming reactants to adversely effect either the chemical additive itself, the properties of the subsequently formed foam, or the processing of the foam. This generally limits both the nature and the amount of chemical additive that can be incorporated into the polyurethane foam. Another disadvantage is that in situ impregnation is not useful with a volatile or heat-sensitive chemical additive, for the heat of the urethane polymerization reaction will degrade the additive or will volatilize and drive the additive out of the foam-forming mixture.
Consequently, it is an object of this invention to provide a process for the modification of polyurethane foam by interpenetrating the foam with a chemical additive after it has been formed. It is a further object of this invention to provide a process for increasing the electrical conductivity of the polyurethane foam by interpenetrating the foam with a suitable chemical additive after it has been formed.